The present invention relates generally to the field of geophysical exploration and more particularly to a method for enhancing the interpretability of seismic data by reducing distortion, or smearing, of seismic reflection event amplitudes resulting from residual normal moveout.
In geophysical exploration, normal moveout (NMO) correction of seismic data is generally employed as a precursory step to combining common midpoint (CMP) gathers of seismic signals by summing or stacking. Summing of NMO corrected seismic signals is a powerful technique for suppressing noise in the seismic data. Additionally, summing of NMO corrected seismic signals emphasizes or constructively reinforces the amplitude of corresponding reflection events in the seismic data which are assumed to follow hyperbolic traveltime loci in the CMP gathers of the seismic signals as described generally by Yilmaz in "Seismic Data Processing," Investigations in Geophysics No. 2 (SEG, 1987), pgs. 154-239.
If the traveltime loci of corresponding reflection events in a seismic data deviate from hyperbolic traveltimes, the seismic data, after NMO correction, are said to include residual normal moveout which can manifest itself in distorted or smeared reflection event amplitudes in the CMP summed signals. Consequently, reflection events exhibiting non-hyperbolic moveouts, such as those associated with complex structures, will not be properly mapped in velocity space and subsequent inverse mapping to offset space will result in distorted or smeared reflection event amplitudes As such, residual moveout is often characterized as an error in selecting the "proper" stacking velocity.
For low to moderate frequency content seismic data, summation of corresponding reflection event amplitudes in CMP gathers of seismic signals along assumed hyperbolic traveltime loci generally produces acceptable seismic sections for evaluating subsurface structure. However, even small amounts of residual NMO can distort or smear reflection event amplitudes with the most adverse effects manifesting themselves in seismic data having higher frequency contents. This distortion or smearing of reflection event amplitudes can be especially disabilitating to geophysical interpreters in higher frequency content seismic data.
Recently, Yilmaz in "Velocity-Stack Processing," Geophysical Prospecting 37 (1989), pgs. 357-382, described a technique to reduce reflection event amplitude smearing or distortion by "stretching" reflection event amplitudes assumed to be along hyperbolic traveltime loci to those along parabolic traveltime loci to obtain better resolution for identifying the "proper" stacking velocity for NMO correction and CMP summing. Nevertheless, such method is still premised upon the theoretical assumption that the loci of two-way traveltimes for corresponding reflection events in a CMP gather of seismic signals are hyperbolic and as such do not directly attempt to correct residual normal moveout resulting from corresponding reflection events following non-hyperbolic loci of traveltimes.
Consequently, as exploration geophysical techniques have evolved toward the collection of higher frequency content seismic data and the use of true amplitude seismic data for both structural and stratigraphic interpretation of the earth's subsurface formations, the need to correct residual NMO distortion or smearing of reflection event amplitudes has become a more significant concern to exploration geophysicists.